If youre serious about it then why not go with an iMac, unless portability is really needed. If you were to buy a baseline MBP it would cost $1249, plus another $1000 for the Thunderbolt Display. Thats $2249.
Meanwhile, for about the same amount ($50 more) you can get a 27 iMac with the upgrade to a 3.4GHz i7 (much better then then 2.4 i5) and the 6970m GPU with 2GB of VRAM which is above and beyond anything the MBPs GPU could offer.
With that said, if youre just doing basic iMovie stuff anything will do, I can use iMove 11 just fine on my 2008 macbook. If thats all your doing though the Thunderbolt display is expensive overkill. A decent 1080p monitor can be had for under $400.